Abstract: Temperature readings from a thermistor 16 can be recorded in a counter 20 by means of a self-balancing bridge 17 when gate 15 is open for the same four hours of each day. Clock 10 is set to initiate the four hour period at say 06.00 hrs. so that the body temperature measurement is taken after waking but before rising. Measurements for the first three days of the menstrual cycle are rejected (19) and for the next eight days are accummulated in a store 22. These measurements are subject to a diurnal correction 21 to reduce them to the temperature at the beginning of the 4 hour period. The mean of eight readings is determined (23) and subsequent readings compared (24) with the mean. A Cusum statistical test is then applied to determine the beginning of the infertile period signalled by a rise in body temperature and produce a corresponding display 27.